Rubber composition for tire tread

ABSTRACT

A rubber composition for a tire tread including 100 parts by weight of a diene-based rubber and 5 to 150 parts by weight of carbon black having a nitrogen adsorption specific area N 2 SA (m 2 /g) of 80 to 150 and satisfying a relationship between a TINT (%) and dibutyl phthalate absorption DBPA (ml/100 g) of TINT (%)&gt;1.4 DBPA (ml/100 g) and capable of increasing the frictional force without impairing the hysteresis loss.

TECHNICAL FIELD

The present invention relates to a rubber composition for a tire tread,more specifically, it relates to a rubber composition for a tire treadcapable of increasing the frictional force, without substantiallyimpairing the hysteresis loss.

BACKGROUND ART

As rubbers for a tire tread, rubbers having a high frictional force havebeen sought from the viewpoint of safety. On the other hand, a tirehaving a small rolling resistance, that is, rubbers having a smallhysteresis loss at the time of tire rolling have been sought from theviewpoint of the environment and health. To achieve both of theseperformances, the technology for compounding silica is known (forexample, see Journal of the Adhesion Society of Japan, Vol. 37, No. 5(2001), pp. 21 to 26). As the reason why silica exhibits such acharacteristic, the fact that a silica-containing rubber has a lowmodulus of elasticity at the low strain region may be mentioned.However, since silica has to be chemically bonded with rubber using asilane coupling agent, the reaction between the silica and the silanecoupling agent has to be controlled. In particular, there is a problemthat restrictions arise in processing when compounding a large amount ofsilica.

DISCLOSURE OF INVENTION

Accordingly, the object of the present invention is to provide a rubbercomposition for a tire tread capable of increasing the frictional force,without substantially lowering the hysteresis loss.

In accordance with the present invention, there is provided a rubbercomposition for a tire tread comprising 100 parts by weight of adiene-based rubber and 5 to 150 parts by weight of carbon black having anitrogen adsorption specific area N₂SA (m²/g) of 80 to 150 andsatisfying a relationship between a tinting strength (vs. ITRB) (i.e.,TINT) (%) and dibutyl phthalate absorption DBPA (ml/100 g) of TINT(%)>1.4 DBPA (ml/100 g).

In accordance with the present invention, there is also provided arubber composition for a tire tread comprising 100 parts by weight of adiene-based rubber having an average glass transition temperature Tg inthe range of −50° C. to −10° C., 5 to 150 parts by weight of carbonblack having a nitrogen adsorption specific area N₂SA (m²/g) of 80 to150 and a dibutyl phthalate absorption DBPA (ml/100 g) in the range of30 to 80 and 0 to 145 parts by weight of any other optional filler.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, the nitrogen adsorption specific area N₂SA(m²/g) means the value measured using a Model 2300 automatic specificsurface area measurement instrument according to the method of JISK6217-2, the tinting strength (vs. ITRB) (i.e., TINT) (%) means thevalue measured under conditions of the film stretching method using aDensichron reflectometer according to the method of JIS K6217-5 and thedibutyl phthalate absorption DBPA (ml/100 g) means the value measuredusing an Absorptometer Model B according to the method of JIS K6217-4.

The inventors proceeded with studies on the types of carbon blackcompounded into the diene-based rubber and, as a result, found that, bycompounding carbon black having a suitable particle size and having ahigher coloring degree compared with the magnitude of the structure, itis possible to surprisingly obtain a rubber composition having a modulusof elasticity at the low strain region of about the same extent as inthe case of compounding silica.

The diene-based rubber capable of blending the rubber composition for atire tread according to the present invention includes, for example, anydiene-based rubbers which can be used as a starting rubber for a tire.As representative diene-based rubbers, various types of natural rubbers(NR), various types of polyisobutylene rubbers (IR), various types ofpolybutadiene rubbers (BR), various types of styrene-butadiene copolymerrubbers (SBR), ethylene-propylene-diene terpolymers (EPDM), etc. may bementioned. These rubbers may be used alone or in any mixture thereof.The diene-based rubber used in the present invention having an averageglass transition temperature of preferably −50° C. to −10° C., morepreferably −45° C. to −20° C. is particularly preferable from theviewpoint of improvement of the frictional force.

The rubber composition for a tire tread according to the first aspect ofthe present invention, as explained above, includes 5 to 150 parts byweight, particularly 10 to 140 parts by weight, of carbon black having anitrogen adsorption specific area N₂SA (m²/g) of 80 to 150, preferably82 to 140, and satisfying a relationship between a TINT (%) and dibutylphthalate absorption DBPA (ml/100 g) of TINT (%)>1.4 DBPA (ml/100 g).

The nitrogen adsorption specific area N₂SA (m²/g) of the carbon black isa value representing the particle size of the carbon black and ismeasured according to JIS-K6217. The coloring degree TINT (%) is animportant indicator for carbon black when using for, for example colorink. It shows the brightness when coating carbon black mixed togetherwith a white pigment (measured by JIS-K6217). The darker the color, thegreater the coloring power. The DBPA (ml/100 g) is a value representingthe structure of the carbon black particles and is measured according toJIS-K6217.

If the particle size of the carbon black used in the present invention,that is, the value of N₂SA, is too small, the characteristics at breakor abrasion resistance of the rubber composition obtained isinsufficient, while conversely if too large, it is difficult to make thecarbon black sufficiently disperse at the time of mixing with the rubberetc., and therefore, the characteristics at break or abrasion resistancebecome insufficient. The carbon black used in the present invention hasa DBPA of preferably 30 to 80 (ml/100 g), more preferably 40 to 78(ml/100 g). It is possible to much more increase the frictional forceusing this range of DBPA. Note that it is also possible to use silicatogether with the carbon black to an extent not causing problems inprocessing.

The carbon black used in the present invention has to have a TINT (%) ofmore than 1.4 times the value of the DBPA (ml/100 g), preferably atleast 1.5 times, more preferably at least 1.6 times. As the carbon blackhaving a large coloring degree compared with the magnitude of thestructure, for example, carbon black which had been used for ink in thepast may be mentioned. This carbon black differs, in surface activity,from conventional carbon black for rubber use and therefore, a tightbound rubber is not formed after blending. Further, there are littlestrongly constrained rubber molecules, and therefore, the modulus ofelasticity at the low strain region becomes lower in the same way as inthe case of compounding silica. Further, if the structure of the carbonblack is small, the carbon black will become difficult to form a networkin the rubber and the modulus of elasticity at the low strain regiontends to be further decreased.

Explained further, the carbon black used for tire treads in the past wasconsidered preferable if having a small particle size (i.e., large N₂SA)and large structure (i.e., large DBPA). This was because it wasconsidered that such a carbon black formed a lot of tight bound rubberand exhibited a high reinforcing property. Contrary to this, the carbonblack used in the present invention has a suitable particle size (i.e.,N₂SA of 80 to 150 m²/g) and a small structure (i.e., DBPA of 30 to 80ml/100 g) and could not be used for tire treads up to now. Further,regarding the TINT, there had been carbon black having a large DBPA anda high TINT, but carbon black having a high TINT and a small DBPA wasnever used for tire treads. That is, the carbon black used in thepresent invention was never used for tire tread applications and iscompletely novel for tire tread use. In this way, carbon black having asmall structure had not been preferred from the viewpoint of thereinforcability, but the inventors found that, by using carbon blackhaving a suitable particle size and high TINT, it is possible to obtaina rubber composition superior in wet performance, even compared withconventional carbon black, and sufficient for tire tread use even interms of reinforcability.

In the second aspect of the present invention, by adding to 100 parts byweight of a diene-based rubber having a Tg of −50° C. to −10° C.,preferably −45 to −20° C., carbon black having an N₂SA of 80 to 150m²/g, preferably 82 to 140 m²/g and a DBPA of 30 to 80 ml/g, preferably40 to 78 ml/100 g, it is possible to obtain the desired effect of thepresent invention regardless of the TINT of the carbon black. Note thatthe rubber composition according to the second aspect of the presentinvention may include, in addition to the 100 parts by weight ofdiene-based rubber and 5 to 150 parts by weight of the above specifiedcarbon black, 0 to 145 parts by weight, preferably 5 to 135 parts byweight, of any optional fillers (for example, general carbon black orsilica).

The rubber composition according to the present invention may havecontain, in addition to the above essential ingredients, variousadditives generally used for tires, such as vulcanization andcross-linking agents, vulcanization and cross-linking accelerators,various types of oils, anti-aging agents, and plasticizers. Theformulations may be mixed by a general method to form the compositionswhich may then be used for vulcanization or cross-linking. The amountsof these additives may be made the conventional general amounts, so faras the object of the prevent invention is not adversely affected. Therubber composition of the present invention is useful as a rubbercomposition for a tire tread having the increased frictional force,without impairing the hysteresis loss.

EXAMPLES

The present invention will now be explained further by Examples, but thescope of the present invention is, of course, not limited to theseExamples.

Examples 1 to 4 and Comparative Examples 1 and 2

Preparation of Samples

Following each of the formulations shown in Table I and using a 16-literinternal Bambury mixer, the rubber and the carbon black and othercompounding agents, other than the sulfur and vulcanization accelerator,were mixed for 5 minutes to obtain a master batch, then an open roll wasused to mix the vulcanization accelerator and sulfur therein to obtainthe rubber composition. Each rubber composition thus obtained wasvulcanized in a 15×15×0.2 cm mold at a temperature of 160° C. for 30minutes to obtain a vulcanized rubber sheet.

Next, each vulcanized rubber sheet thus obtained was measured for ΔE′(20° C.) and wet braking performance according the following methods.The results are shown in Table I.

Measurement of ΔE′

Using a viscoelasticity spectrometer made by Toyo Seiki, the storagemodulus E′ and the loss modulus E″ were measured using dynamic strainfrom 0.2% to 8.2% as a variable. The modulus of elasticity E′₀ at a zerostrain and the modulus of elasticity E′_(∞) at an infinite strain werefound by a Cole-Cole plot (see G. Kraus, Reinforcement of Elastomers,Interscience Publishers, p. 81 (1965)) and the value of E′₀-E′_(∞) wasmade ΔE′. The smaller the value of this ΔE′, the better thefollowability of protuberance on the road surface and the more improvedthe grip force.

Wet Braking Test

Size 195/65R15 tires using different rubber compositions for the treadparts were fabricated and measured for a braking distance on an asphaltroad surface from an initial speed of 100 km/h. The results were indexedto Comparative Example 1 as 100. The larger the figure, the shorter thebraking distance and therefore the better.

TABLE I Comp. Comp. Ex. 1 Ex. 1 Ex. 2 Ex. 2 Ex. 3 Ex. 4 Formulation(parts by weight) Nipol 9528R*¹ 68.75 68.75 68.75 68.75 68.75 68.75Nipol 1502*² 50 50 50 50 50 50 N234*³ 80 — — 40 — — Nipsil AQ*⁴ — — — 4020 20 RCF#44*⁵ — 80 — — 60 — RCF#45L*⁶ — — 80 — — 60 Santoflex 1 1 1 1 11 6PPD*⁷ Zinc oxide 3 3 3 3 3 3 no. 3*⁸ Stearic acid*⁹ 1 1 1 1 1 1Aromatic oil*¹⁰ 16.25 16.25 16.25 16.25 16.25 16.25 Santocure 2 2 2 2 22 NS*¹¹ Sulfur*¹² 1.7 1.7 1.7 1.7 1.7 1.7 Evaluated properties ΔE′ (20°C.) 35.9 25.9 23.2 32.6 25.7 21.7 Wet braking 100 108 106 105 110 107index Footnotes of Table I *¹27.3% oil extended SBR (Tg = −35° C.) madeby Nippon Zeon *²SBR (Tg = −51° C.) made by Nippon Zeon *³Carbon blackmade by Showa Cabot (see Table II) *⁴Silica made by Nippon SilicaIndustrial *⁵Carbon black made by Mitsubishi Chemical (see Table II)*⁶Carbon black made by Mitsubishi Chemical (see Table II) *⁷Antiagingagent made by FLEXSYS *⁸Industrial use zinc oxide made by Seido Chemical*⁹Stearic acid made by Nippon Oil and Fat *¹⁰Aromatic oil made by ShowaShell Oil Sekiyu *¹¹Sulfenamide-based vulcanization accelerator made byFLEXSYS *¹²Sulfur made by Tsurumi Chemical

TABLE II N234 RCF#44 RCF#45L N₂SA (m²/g) 112 99.7 117 DBP (ml/100 g) 12176 54 1.4 × DBP 169 106 76 TINT (%) 122 129 141

Note that the average Tg of Nipol 9528R (Tg=−35° C.) and Nipol 1502(Tg=−51° C.) is −35×0.5+(−51×0.5)=−43° C.

INDUSTRIAL APPLICABILITY

As explained above, according to the present invention, it is possibleto increase the frictional force of rubber, without causing adeterioration of the hysteresis loss, and therefore the composition ispreferable for use as a rubber composition for a tire tread.

1. A rubber composition for a tire tread comprising (i) 100 parts byweight of a diene-based rubber having an average glass transitiontemperature Tg of −50° C. to −10° C. and wherein said diene-based rubberis at least one member selected from the group consisting ofpolybutadiene rubber (BR) and styrene-butadiene copolymer rubber (SBR)and (ii) 5 to 150 parts by weight of carbon black having a nitrogenadsorption specific area N₂SA (m²/g) of 80 to 150 and a dibutylphthalate absorption DBPA in the range of 30 to 80 (ml/100 g) andsatisfying a relationship between a tinting strength TINT (%) and adibutyl phthalate absorption DPBA (ml/100 g) of TINT %>1.5 DPBA (ml/100g) and (iii) 0 to 145 parts by weight of any other optional filler.
 2. Apneumatic tire using a rubber composition according to claim 1 for atire tread.
 3. A rubber composition for a tire tread, as claimed inclaim 1, wherein the DBPA is in the range of 40 to 78 ml/100 g.
 4. Arubber composition for a tire tread, as claimed in claim 1, wherein theaverage glass transition temperature Tg of said diene-based rubber is inthe range of −45° C. to −20° C.
 5. A rubber composition for a tiretread, as claimed in claim 4, wherein the DBPA is in the range of 40 to78 ml/100 g.
 6. A rubber composition for a tire tread, as claimed inclaim 1, wherein the diene-based rubber is styrene-butadiene copolymerrubber (SBR).